In many industries, such as the aerospace industry, one of the effective ways to reduce the weight of an aircraft is to reduce the density of aluminum alloys used in the aircraft's construction. It is known in the art that aluminum alloy densities may be reduced by the addition of lithium. However, lithium in aluminum-based alloys also raises other problems. For example, the addition of lithium to aluminum alloys may result in a decrease in ductility and fracture toughness. For use as aircraft structural parts, it is obviously imperative that any alloy have excellent fracture toughness and strength properties.
Various aluminum-lithium alloys have been registered with the Aluminum Association. For example, alloys AAX2094 and AAX2095, registered in 1990, include alloying elements of copper, magnesium, zirconium, silver, lithium and inevitable impurities.
U.S. Pat. No. 5,032,359 to Pickens et al , issued Jul. 16, 1991, discloses an improved aluminum-copper-lithium-magnesium-silver alloy possessing high strength, high ductility, low density, good weldability and good natural aging response. Typically, these alloys consist essentially of 2.0-9.8 wt.% of an alloying element which may be copper, magnesium, or mixtures thereof, the magnesium being at least 0.01 wt.% with about 0.01-2.0 wt.% silver, 0.05-4.1 wt.% lithium, and less than 1.0 wt.% of a grain refining additive which may be zirconium, chromium, manganese, titanium, boron, hafnium, vanadium, titanium di-boride or mixtures thereof.
Another prior art alloy for use in aircraft industry application is disclosed in U.S. Pat. No. 4,648,913 to Hunt, Jr. et al. In this patent, an aluminum-based alloy is disclosed comprising 0.5-4.0 wt.% lithium, 0-5.0 wt.% magnesium, up to 5.0 wt.% copper, 0-1.0 wt.% zirconium, 0-2.0 wt.% manganese, 0-7.0 wt.% zinc, 0.5 wt.% maximum iron, 0.5 wt.% maximum silicon, the balance aluminum and incidental impurities. This alloy is subjected to heat treating and working steps to improve strength and toughness characteristics. The heat treating and working steps of Hunt, Jr. et al are representative of a T8 temper designation, that is well known to those skilled in the art, which includes solution heat treatment followed by strain hardening and then artificial aging. Related patents include U.S. Pat. Nos. 4,797,165 and 4,897,126 to Bretz et al and 4,961,792 to Rioja et al.
Despite the years of developmental effort, these newly commercialized aluminum-lithium alloys have been selected for relatively few commercial applications. One of the reasons for such a limited commercial success of these aluminum-lithium alloy products is that aluminum-lithium alloys in wrought product form tend to develop very high texture which adversely affects the mechanical properties of the wrought product in the transverse direction. These mechanical property limitations often prevent the implementation of aluminum-lithium alloys in full scale commercial aircraft structural applications.
While poor mechanical properties such as ductility in aluminum-lithium wrought products in the transverse direction are typical for all forms of wrought products, the poor transverse ductility and/or strength are especially prominent in aluminum-lithium extrusions. These ductility and/or strength deficiencies are especially pronounced in extruded product having axisymmetric cross sections.
In the aforementioned Hunt, Jr. et al. '913 patent and related U.S. Pat. Nos. 4,790,884 to Young et al. and 4,861,391 to Rioja et al., solution heat treatment, stretching and aging steps are disclosed to improve various mechanical properties in aluminum-lithium alloys. In the Hunt, Jr. et al patent, the solution heat treated and quenched product is subjected to a single stretching step in an amount greater than a 3% stretch or a working effect equivalent to stretching greater than 3%. However, these types of T8 temper practices are deficient in providing acceptable mechanical properties in the transverse direction. As will demonstrated herein below, unacceptable levels of ductility and strength are evident using these types of conventional practices.
As such, a need has developed to provide improved processing techniques to achieve high strength and ductility in aluminum-lithium alloy wrought products to facilitate their use in aircraft structural applications.
In response to this need, the present invention provides a method of improving the mechanical properties of aluminum-lithium alloys in the transverse direction by imparting a plurality of stretching steps between solution heat treating and aging. None of the prior art discussed above teaches or fairly suggests improving transverse direction mechanical properties in these types of alloys by modifying conventional T8 temper practice in this manner.
The patent to Rioja et al discussed above teaches a two-step aging method for aluminum-lithium alloys. One or both of the aging steps may be preceded by a stretching step in an amount between about 1 to 8 percent.
In the aforementioned patent to Young et al, a method is disclosed for making aluminum-lithium alloy flat rolled product capable of being stretched without the formation of luders lines. In this method, the flat rolled product is preaged prior to stretching. Optionally, a controlled cold working may be employed after solution heat treating and prior to the thermal preaging treatment.
Neither of the patents to Young et al nor Rioja et al teach or fairly suggest improving transverse direction mechanical properties in aluminum-lithium alloy wrought product using a multiple step stretching sequence between the steps of solution heat treating and quenching the wrought product and aging to a predetermined strength level.